Aging is associated with impaired regulation of the immune system, which contributes to the increased incidence of infectious, inflammatory and neoplastic diseases observed in elderly subjects as well as their prolonged post-illness recovery periods. In addition, these changes were shown to be predictive of morbidity and mortality in animal models and humans. While all cells of the immune system contribute to the impaired immunity of old age, T cells are the main contributors, with age related changes reported in both in vivo and in vitro measures of T cell function across all species including humans. Calorie restriction (CR) has been shown to affect many age sensitive immunological responses in animal models, but information related to the effects of CR on immune response of humans is lacking. Preliminary results from the pilot phase of the two-step NIA-supported multi-center clinical trial to determine the biological effects of CR in humans [Comprehensive Assessment of Long-Term Effects of Reducing Intake of Energy (CALERIE) Phase 1], showed that CR significantly improved delayed type hypersensitivity skin response, as well as T cell proliferation, in humans while decreasing production of T cell suppressive factor, PGE2. Thus, we hypothesize that long term CR intervention in adult subjects will enhance the immune response, as indicated by improved T cell-mediated function and reduced production of inflammatory mediators. Furthermore, we hypothesize that these CR-mediated effects on T cells are due to decrease in PGE2 production and/ or intrinsic changes in T cells. We propose to test this hypothesis utilizing subjects enrolled in the second phase of the NIA supported multi-center randomized controlled clinical trial, CALERIE Phase 2. The overall aim of CALERIE Phase 2 is to test the effects of 2 year, 25% CR from baseline energy intake, in adult subjects, on physiology, metabolism, body composition, risk factors for age-related pathologies, and its potential adverse effects. The specific aims of this project are to determine the effect of 2 y, 25% CR on T cell-mediated functions of adult subjects, as well as to explore its underlying mechanisms. Specifically the effect of CR on immune cell profile, proliferate ability of specific T cell subsets, intracellular and extracellular IL-2, IFNy, as well as PGE2 production, will be evaluated before, and following 1 and 2 years of 25% CR. The results from this study will be the first documentation of the impact of CR on immune response in humans, a biologically meaningful and clinically relevant marker shown to be sensitive to CR in various animal models. The proposed studies will also explore the underlying mechanisms of CR-induced modulation of the immune response and will add valuable information to the parent CALERIE Phase 2 studies by providing insight into the cellular and molecular mechanisms of CR induced health effects.